1. Technical Field
The present invention relates to a nonaqueous electrolyte secondary battery. Particularly, the present invention relates to a nonaqueous electrolyte secondary battery using a positive electrode charged at a high electric potential higher than 4.3 V based on lithium in which the amount of a generated gas is small even when the battery is continuously charged at higher temperatures, and the impact safety and reliability thereof are high.
2. Related Art
With the rapid spread of portable electronic equipment, the specifications required of the batteries used in such equipment have become more stringent every year, and there is particular demand for batteries that are compact and thin, have high capacity and superior cycling characteristics, and give stable performance. In the field of secondary batteries, attention is focusing on lithium nonaqueous electrolyte secondary batteries, which have high energy density compared with other batteries. These lithium nonaqueous electrolyte secondary batteries are winning an increasingly large share of the secondary battery market.
FIG. 1 is a perspective view showing a related-art cylindrical nonaqueous electrolyte secondary battery by sectioning the battery perpendicularly. This nonaqueous electrolyte secondary battery 10 uses a wound electrode body 14 produced by winding a positive electrode 11, a separator 13 and a negative electrode 12 which are laminated in this order, and is constituted by a method including: disposing insulating plates 15 and 16 respectively on the top side and bottom side of the wound electrode body 14 to prepare a parts set; holding the parts set in the inside of a steel-made cylindrical battery outer packaging can 17 serving also as a negative electrode terminal; welding not only a power collecting tab 12a of the negative electrode 12 to an inside bottom of the battery outer packaging can 17, but also a power collecting tab 11a of the positive electrode 11 to a bottom plate of a current-intercepting opening-sealing body 18 with a built-in safety device; pouring a predetermined nonaqueous electrolyte through an opening of the battery outer packaging can 17; and sealing the battery outer packaging can 17 with the current-intercepting opening-sealing body 18. Such a nonaqueous electrolyte secondary battery has such an excellent effect that battery performance and reliability are high. Further, the current-intercepting opening-sealing body 18 is a part cutting an electric connection between the coil-shaped electrode body 14 and an outside of the battery according to the pressure elevation inside the battery and the battery has such a constitution that once the electric connection has been cut, the electric connection cannot be recovered even when an internal pressure is lowered.
As a negative electrode active material used in the nonaqueous electrolyte secondary battery, carbonaceous materials such as graphite and an amorphous carbon are widely used, since carbonaceous materials have such excellent properties as high safety because dendrites do not grow therein, while having a discharge potential comparable to that of lithium metal or lithium alloy; excellent initial efficiency; advantageous potential flatness; and high density.
Further, as a nonaqueous solvent of a nonaqueous electrolyte liquid, carbonates, lactones, ethers and esters are used individually or in combination of two or more thereof. Among them, particularly carbonates having a large dielectric constant and having a large ion conductivity, thus the nonaqueous electrolyte liquid thereof are frequently used.
On the other hand, as a positive electrode active material, a lithium-transition metal compound oxide such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), spinel-type lithium manganese oxide (LiMn2O4) and lithium iron oxide (LiFeO2) is used. It is known that by using such a positive electrode in combination with a negative electrode consisting of a carbon material, a 4V-class nonaqueous secondary battery having a high energy density can be obtained. Among them, particularly because of various battery properties more excellent than those of other materials, lithium cobalt oxide and different metal elements-added lithium cobalt oxide are frequently used. However, since not only is cobalt expensive, but also the existing amount of cobalt as a resource is small, for continued use of lithium cobalt oxide as a positive electrode active material of the nonaqueous electrolyte secondary battery, it is desired to make the nonaqueous electrolyte secondary battery having even higher performance and longer life.
For making a nonaqueous electrolyte secondary battery in which lithium cobalt oxide is used as a positive electrode active material, having even higher performance and longer life, it is an essential task to enlarge the capacity and energy density of the battery and improve the safety of the battery. Among them, as a method for enlarging the capacity of the battery, enlarging the density of an electrode material, making a power collector and a separator to be a thin film and enlarging the charging voltage of the battery voltage, are generally known. Among them, enlarging the charging voltage of the battery voltage is a useful technology as a method capable of enlarging capacity without changing the constitution of the battery and is an essential technology for enlarging the capacity and energy density.
In a nonaqueous electrolyte secondary battery using a lithium-containing transition metal oxide such as lithium cobalt oxide as a positive electrode active material and using a carbon material such as graphite as a negative electrode active material, the charging voltage is generally 4.1 to 4.2 V (the electric potential of the positive electrode is 4.2 to 4.3 V based on lithium). Under such a charging condition, only 50 to 60% of the theoretical capacity of the positive electrode active material is utilized in. Therefore, when the charging voltage can be enlarged more, 70% of the theoretical capacity of the positive electrode can be utilized, thereby enabling capacity and energy density of the battery to be enlarged.
For example, JP-A-2005-85635 discloses an invention of a nonaqueous electrolyte secondary battery using a positive electrode active material in which a zirconium-containing compound is attached to the surface of lithium cobalt oxide particles, and capable of achieving advantageous charging/discharging cycle property even when the battery is charged at a high voltage of 4.3 to 4.4 V based on lithium.
Further, JP-A-2005-317499 discloses an invention of a nonaqueous electrolyte secondary battery using a mixture of lithium cobalt oxide and a layer-shaped lithium nickel cobalt manganese oxide to which different metal elements are added as a positive electrode active material, and capable of being stably charged at a high charging voltage. This positive electrode active material is produced so that by adding different metal elements of at least Zr and Mg to lithium cobalt oxide, the structural stability thereof at a high voltage is improved and further, by incorporating a layer-shaped lithium nickel cobalt manganese oxide having high thermal stability at a high voltage, the safety is secured. By using a combination of a positive electrode using the above positive electrode active material and a negative electrode having a negative electrode active material consisting of a carbon material, a nonaqueous electrolyte secondary battery capable of achieving advantageous cycle property and thermal stability even when the charging voltage is a high voltage of 4.3 V or more, has been obtained.
However, when the state of charge of the positive electrode active material is enhanced by further enhancing the charging potential of the nonaqueous battery, the decomposition of an electrolyte on the surface of the positive electrode active material and a structural deterioration of the positive electrode active material itself tend to occur. Such a structural deterioration of the positive electrode active material and decomposition of the electrolyte increase according to the increase of the charging voltage, which leads to the lowering of thermal stability, so that for providing a nonaqueous electrolyte secondary battery which has a large capacity and is capable of maintaining safety compared to that of a conventional battery having a charging voltage of around 4.2 V, there is much room for improvement.
On the other hand, as a technology for enhancing the safety of the nonaqueous electrolyte secondary battery, technologies shown in JP-A-7-220759, JP-A-2006-310010, JP-T-2001-501355 and JP-A-2005-038722 are also known. Specifically, in JP-A-7-220759, there is shown an example in which for the purpose of preventing the short circuit due to an active material which has been eliminated during the production of the nonaqueous electrolyte secondary battery, a porous protecting film having a thickness of 0.1 to 200 μm and consisting of insulating fine particles such as alumina, silica, polyethylene and the like, and a resin binder is formed on the surface of a negative electrode active material-applied layer or a positive electrode active material-applied layer.
In JP-A-2006-310010, there is shown an example in which for achieving a stabilized cycle life, in a nonaqueous electrolyte secondary battery in which a negative electrode has a width/length larger than that of a positive electrode, for the purpose of preventing the short circuit due to dendrites of lithium caused on a terminal face of the negative electrode while repeating the charging and discharging, a porous protecting film consisting of an inorganic oxide filler and a binder is formed on the surface of the positive electrode and/or negative electrode.
In JP-T-2001-501355, there is shown an example in which in an alkali metal ion secondary battery, for the purpose of preventing the short circuit due to the formation of dendritic crystals (dendrites) during the charging, fluoroethylene carbonate and propylene carbonate are incorporated in the nonaqueous electrolyte.
Further, in JP-A-2005-038722, there is shown an example using a nonaqueous electrolyte liquid in which a fluorinated cyclic ester such as fluoroethylene carbonate is added into a nonaqueous electrolyte containing a cyclic carbonate and γ-butylolactone. In the invention disclosed in JP-A-2005-038722, on the surface of the negative electrode, a stabilized film of a fluorinated cyclic ester is formed and by this film, the decomposition of the electrolyte is suppressed, so that not only the cycle property of the battery can be improved, but also the generation of a gas during the storage of the battery at higher temperatures can be suppressed. However, it is described that during an overcharging of the battery, the fluorinated cyclic ester is decomposed and a gas is generated, so that a safety valve can be quickly operated.
As already described above, the nonaqueous electrolyte secondary batteries disclosed in JP-A-7-220759 and JP-A-2006-310010 can suppress the short circuit between the electrodes due to the formation of dendrites during the charging. However, there is no description indicating suppressing the generation of a gas caused by the decomposition of the electrolyte liquid.
The nonaqueous electrolyte secondary battery disclosed in JP-T-2001-501355 can at least suppress the short circuit between the electrodes due to the formation of dendrites during the charging. However, since fluoroethylene carbonate is likely to be reduced, it has the disadvantage that the negative electrode decomposes, thereby generating carbonic acid gas and an organic gas.
Also in the nonaqueous electrolyte secondary battery disclosed in JP-A-2005-038722, since during an overcharging, by making positive use of the decomposition of an added fluorinated cyclic ester such as fluoroethylene carbonate, a safety valve is caused to be quickly operated, a large amount of gas is generated inside the battery. Such generation of a large amount of gas causes a battery swell in a prismatic battery and of an action of a current blocking mechanism in a cylindrical battery, which leads to the lowering of the reliability of the nonaqueous electrolyte secondary battery in the market.
Particularly, in the case of the positive electrode in which the charging is performed at a high electric potential of 4.4 V or more based on lithium, also on the surface of the positive electrode, not only fluoroethylene carbonate, but also dimethyl carbonate (DMC) as another nonaqueous solvent component are likely to be decomposed, so that a larger amount of gas is generated. Moreover, in a nonaqueous electrolyte secondary battery using the positive electrode in which the charging is performed at a high electric potential of 4.4 V or more based on lithium, the positive electrode active material itself is poor in thermal stability, so that there is such a disadvantage that such a nonaqueous electrolyte secondary battery is poor in impact resistance.